The present invention relates to the exploration and production of petroleum from earth formations, and more particularly, to methods for determining the amount of fluids present in such a formation.
In the petroleum industry, one of the most valuable and informative techniques for determining the characteristics of an earth formation located well below the surface, and the nature of the fluids which it may contain, is to remove and bring a portion of the formation and/or its fluids to the surface for analysis. This is done most commonly by "coring" the formation. This coring may be accomplished by conventional coring, pressure coring or sponge coring, when it is desired to recover a substantial portion of a formation or formations. However, sidewall plugs or cores are also employed, when only a small portion of the earth formation is desired or when more economical samples are desired.
It is often important to type the hydrocarbon crude in order to be able to determine the ease of removal of the crude oil. A reliable method for estimating key hydrocarbon properties, such as API gravity, from sidewall samples could preclude the need for expensive production testing. The potential economics through reduced costs and/or reduced risks to a well could be substantial. Further, variations in hydrocarbon properties within or between reservoirs could be more easily determined with increased sample density at lower cost.
The importance of coring in the production of petroleum has recently been increasing as more and more secondary and tertiary recovery is being made of petroleum reserves. In a formation undergoing primary production, the original reservoir fluids are little altered from their condition for the last several thousand or more years. They may migrate as the oil is produced, but their properties are not changed significantly. However, when fluids and/or other compounds are injected into a formation to stimulate its production, the nature of the connate fluids is accordingly altered, sometimes to a very substantial extent. When this occurs, the more traditional well-logging tools may be unable to provide any useful information about the formation and/or its fluids. In all too many instances, the only way to determine how much oil is left, and thus whether it can be produced economically, is to physically recover a portion of the formation by taking a core sample.
It will therefore be appreciated that the analysis of the amount and properties (viscosity, API gravity, etc.) of the oil in a core sample can be critically important. The viscosity (which is correlated to API gravity) of the crude in a formation often determines whether the oil in the formation may be commercially produced. Similarly, the final true residual oil saturation of a formation is a determination that can make or break a multi-million dollar enhanced recovery project.
Typically, oil is extracted from a portion of a core by means of a soxhlet extractor. Other techniques are also available such as the Dean Stark extraction technique. However, typically these prior art techniques use a hot solvent which dissolves the oil and boils off the water that is found in the core sample. This is usually the result of using a solvent that is capable of dissolving only the oil and not the water. At the end of the extraction, the oil recovered remains in the solvent and is usually discarded. The soxhlet technique only cleans the sample. In Dean-Stark analysis, the condenser is placed to the side of the extraction vessel's center line and any water that is boiled off is condensed and collected in a side arm under the condenser; the (generally lighter) solvent then floats on top of the collected water and overflows the sidearm to drip back onto the sample. In Dean-Stark analysis the volume of oil is inferred by subtracting the amount of water boiled out of the rock from the total pore volume of the rock. If there was any gas in the original core, the calculated oil volume will be too high.
That is, the prior art techniques can not extract and separately recover both the pure oil (essentially solvent free) and pure brine or water. Without an essentially solvent-free crude extract, it is difficult to make direct measurements of the extract properties. Further, these techniques are unable to predict produced crude properties from a crude extract that may have been adulterated due the coring and/or extraction process.
These and other limitations and disadvantages are overcome by the present invention, however, and methods and apparatus are provided for determining the amount of fluids in a core sample, and the petrophysical properties of those fluids.